Lecture 8 - Janzen connell hypothesis

Question 1

what is a key driver of individual and species turnover in tropical forests?

Answer 1

gap dynamics

Question 2

at what point if competition very intense and what is important at this stage?

Answer 2

the point of forest turnover - when there is death of a tree and creation of a gap in the canopy
- processes that affect composition at this stage are likely to be important

Question 3

what are natural enemies and where are they prevalent?

Answer 3

extremely prevalent in tropical forests - e.g. insects and fungi - major cause of mortality in seeds and seedlings

Question 4

what is tree species richness correlated with?

Answer 4

insect species richness - implied that increase in plant species richness drives evolution of insect speciesn

Question 5

what is the role of natural enemies?

Answer 5

postulated that natural enemies might play a role in maintaining diversity in tropical forests and other hyper diverse systems = JC

Question 6

what is the JC hypothesis?

Answer 6

• all plants are attacked by natural enemies
• many natural enemies are specialists
• specialists will aggregate on high densities of their hosts
• if a species becomes very common it will attract high numbers of enemies
• rare species will attract fewer enemies - hence rare species should increase, common species should become rarer

Question 7

principles of JC

Answer 7

1) dispersal shadows - tropical forests dont have an even distribution of seeds and trees - seeds fall by parent (shadow dispersal) - density of seeds decreases further away from parent
2) aggregation of hosts - when looking at spatial distribution of the seeds, natural events show area around host to have high host density
3) local density dependence - probability of death increases with seed density

Question 8

how do density and survival vary with distance from the parent tree?

Answer 8

at short distances the desnsity of seeds is high and this high density attracts lots of natural enemies - consequently survival is low - opposite for long distances

Question 9

when is survival maximised?

Answer 9

at an intermediate distance - the precise distance depending on strength of density dependence

Question 10

where is the death zone?

Answer 10

immediately beneath the parent tree - essentially no survivors - conspecifics cannot recruit

Question 11

what happens when you vary the strength of density dependence and the extent of dispersal?

Answer 11

effects the precise locations of the death zone and peak of recruitment but overall pattern remains given some assumptions (e.g. leptokurtic dispersal and overcompensating dispersal)

Question 12

what is a population recruitment curve?

Answer 12

seeds x survival

Question 13

describe the net effect on recruitment?

Answer 13

species cannot ‘self-replace’ because intense density dependence kills all seeds/seedlings immediately adjacent to parent tree in the death zone
i.e. when a gap is created it cannot be re-occupied by the same species must be different = DIVERSITY

Question 14

2 predictions of the JC

Answer 14

1) distance dependence - arises because of the relationship between dispersal and distance and modulated by local density dependence, but accumulation of enemies may yield other distance effects
2) rare species advantage - species that become locally abundant are at at disadvantage - rare species attract fewer enemies = advantage - density dependence

Question 15

2 assumptions of the JC

Answer 15

1) needs overcompensating density dependence

2) specialist natural enemies - models with generalist natural enemies yield no rare species advantage